1. Field of the Invention
The present invention relates to tuning of a station, and in particular, but not exclusively, to tuning of modulator means of a station.
2. Description of the Related Art
A station such as a transmitter, a receiver or a transceiver can be used for communication media towards another station. The station may comprises user equipment such as a mobile station of a cellular communication system. Said other station may be another node in the communication system. The other node may comprise user equipment or a base station of a communication system and so on. The communication may be transmitted as analogue or digital signals or a combination of these, such as by digitally-modulated analogue signals.
One or more of the components of a station may introduce an undesired error to the signal produced by the station. This means that the station may produce a signal that does not correctly represent the information that was intended to be communicated by means of the station. The error may be caused, for example, by variations in the components used for the station. Therefore, a procedure known as tuning or calibration is typically required at the end of the manufacturing process of a station. The tuning is accomplished to ensure that the manufactured station meets the quality requirements and/or technical specification set for it.
An error that may need to be corrected by the calibration is the so called offset voltage. The offset voltage may cause an error, often referred to carrier leakage, in the signal output from the station.
The offset voltage may be caused by the processing of baseband signals. This can happen, for example, in I/Q modulators which are generally used in mobile stations or in any other station in which a baseband in-phase signal (I) and quadrature signal (Q) are processed. The quadrature techniques are typically used in modern stations in implementation of modulation of the signals.
The carrier leakage has been conventionally compensated in a transmitter station by tuning the station before it is taken into use. This tuning can be done by utilizing test signals in I and Q branches of the station. These signals are first driven to zero and then DC signals on both branches are subsequently adjusted so that the output power of the station at an output power level detector in minimised. One of said DC signals is then incremented or decremented by small steps and the change in the output power is recorded after each step. This continues until the output power of the modulator reaches a local minimum. The other branch is then selected and tuned by repeating the process. The step size is reduced at this stage until an optimum tuning of the station is reached.
In order to implement the above algorithm the detector needs to have a substantially high sensitivity. Typically, this is obtained by biasing the detector and providing amplification means in the detector circuit. Components required for implementing these functions may increase the complexity of the station. Furthermore, the stepwise operation is substantially time consuming. It might be advantageous, especially in mass production, if the time required by the tuning could be reduced.
The inventors have also found that it might also be advantageous in certain circumstances if a station could be tuned after the station has already been used for a while. For example, changes in the conditions in which the station is used (such as temperature or humidity and so on) may influence the operation of the station. In addition, components of the station may experience ageing, extreme conditions, even some mechanical damage and so on. In order to maintain optimal operation conditions of the station it might thus be advantageous if the station could be tuned even after the station has been in use.